Groupon e-mails a daily deal per city and consumers opt-in to buy a restaurant gift certificate, a health and beauty service, or entertainment offer at typically 50% off face value — say $30 voucher for $60 dinner or $50 for a spa package. Once the deal has hooked a predetermined number of people, everyone who has opted in then "buys" the deal, and the marketer gets the money minus Groupon's cut, which is usually 30%-50%, according to Groupon founder Andrew Mason. It only works if the deal attracts critical mass, however — if the offer doesn't reach its threshold, the deal is canceled and no one pays, not even the provider. (The threshold varies by advertiser; some deals require only 50 people to buy in, others need 500.)